This invention relates to an electronically controlled automatic transmission in which shift solenoids are controlled by a microcomputer in dependence upon a signal indicative of vehicle velocity and a signal indicative of the degree of throttle opening. More particularly, the invention relates to an electronically controlled automatic transmission of the type described in which a signal outputted by a throttle opening sensor is adjusted automatically.
In a microcomputer-controlled automatic transmission having a torque converter with a lock-up clutch, shifting generally is controlled in the manner shown in FIG. 6.
In FIG. 6, an automatic transmission 1 is equipped with a torque converter 2 on its input side. The latter has a housing 3 in which there are provided a pump impeller 4, a turbine runner 5 and a lock-up clutch 6. In the torque converting region of a gearchange pattern, the lock-up clutch 6 is freed from the housing 3, so that power from the engine is transmitted to the automatic transmission through the pump impeller 4 and turbine runner 5. In the lock-up region, the lock-up clutch 6 is engaged with the housing 3, whereby power from the engine is transmitted directly to the automatic transmission via the housing 3.
The automatic transmission 1 comprises a well-known group of planetary gears and a friction engagement unit for locking and releasing the elements of the planetary gears. By selectively actuating friction elements automatically in a hydraulic circuit 7 in dependence upon the traveling condition of the vehicle, the vehicle can be made to travel in an optimum speed. The hydraulic circuit 7 is provided with shift solenoids 8, 9 and a lock-up solenoid 10. On the basis of a combination of on/off signals applied to the shift solenoids 8, 9 to energize or de-energize them, the aforementioned friction elements are selectively actuated to shift speeds. The lock-up clutch 6 is engaged or disengaged based on whether the lock-up solenoid 10 is energized or de-energized. The solenoids 8, 9 and 10 are actuated by an electronic control unit (ECU) 11 in dependence upon vehicle velocity and throttle opening on the basis of a gear-change pattern decided for each and every shift position. More specifically, the ECU 11 includes an input signal processing circuit 15, a CPU 16 and a solenoid driver circuit 17. Output signals from a shift position sensor 12, a velocity sensor 13 and a throttle opening sensor 14 are applied to the CPU 16 via the input signal processing circuit 15. The CPU 16 has a memory storing gear-change patterns and lock-up diagrams. A gear-change pattern and lock-up diagram which correspond to the shift position (e.g. D range or L range) are selected, an optimum speed commensurate with the vehicle velocity and throttle opening is decided on the basis of the selected gear-change pattern, engagement or disengagement of the lock-up clutch 6 is determined based on the lock-up diagram, and an appropriate signal is delivered to the solenoid driver circuit 17 to actuate the solenoids 8, 9 and 10 accordingly.
The aforementioned throttle opening sensor 14 has a throttle opening - output voltage characteristic that exhibits a statistical dispersion resulting from the manufacturing process. In order to reduce a deviation in shift point and lock-up point resulting from this dispersion, the conventional practice is to adjust the mounting position of the throttle opening sensor in such a manner that a predetermined output voltage will be obtained at a throttle opening of 100% (or 0% if desired), and finely adjust the mounting position in such a manner that the output voltage will be suppressed, within the range of tolerance of the throttle cable length, at a throttle opening on the opposite side, namely 0% (or 100%).
When the mounting position of the conventional throttle opening sensor is adjusted on the side where the throttle opening is 100% (or 0%), as shown in FIG. 7, a problem that arises is that the dispersion or variance on the opposite side (namely the 0% or 100% side) cannot be absorbed. In other words, assume that a in FIG. 7 is the designed characteristic of the throttle opening sensor. In such case the throttle opening when the throttle sensor output voltage is V.sub..theta. should be .theta..sub.2 %. However, where the actual throttle sensor characteristic is b, the ECU will will render a determination to the effect that the throttle opening with respect to the output voltage V.sub..theta. is .theta..sub.1 %. Accordingly, whereas a shifting of gears and engagement or disengagement of the lock-up clutch should take place at a shift point and lock-up point corresponding to a throttle opening of .theta..sub.2 %, these occur at a shift point and lock-up point corresponding to the throttle opening of .theta..sub.1 %. As a result, the shift point and lock-up point corresponding to the throttle opening of .theta..sub.2 % deviate from their correct positions.
A similar problem arises when the throttle opening - output voltage characteristic varies due to a change in the length of the throttle cable caused by a variation in temperature, or due to a change in resistance value caused by aging.